Low back disorders (LBDs) remain one of most prevalent, debilitating, and costly occupational health problems in the United States. Mechanical loading on the lumbar spine is a central factor in the causation and prevention of occupational LBDs. The existing knowledge and methodological bases for evaluating mechanical forces on the lumbo-sacral (L5-S1) inter-vertebral disc were largely developed from static and/or in vitro cadaveric studies, or from in vivo dynamic studies that did not determine the internal vertebral movement or disc deformation. They are limited in their applicability in assessing the low back disorder or injury risks associated with dynamic work activities such as manual load lifting. Our overarching hypothesis is that more accurate understanding of the dynamic responses of lumbar spine during load-lifting will lead to more effective prevention and control of LBDs. The objective of this exploratory study is to characterize the relationships between dynamic responses of the lumbar spine and lifting dynamics defined by the load and lifting kinematics (i.e., dynamic postures), and begin to develop an improved model for evaluating the low back mechanical stress associated with manual tasks. The study will take advantage of a state-of-the-art dynamic radiographic system capable of measuring lumbar vertebral kinematics in vivo with unprecedented accuracy, and our extensive experience of studying the biomechanics of load lifting. We will pursue two specific aims: Aim 1 is to characterize the relationship between lumbar vertebral kinematics and dynamic lifting posture and the effect of load lifted on the relationship; we will acquire data of the lumbar vertebral kinematics using the dynamic X-ray system and the whole-body lifting kinematics using a motion capture system during the performance of lifting tasks. Aim 2 is to characterize the dynamic relationship between L5-S1 disc deformation and compressive force distribution; we will create a novel spring-loaded wobbling platform model to characterize this relationship and specify it empirically using data obtained from Aim 1; we will compare the L5-S1 disc compressive forces determined based on the new dynamic relationship and by the conventional approach which does not take into account the dynamic disc deformation. The proposed project is the first attempt to investigate the truly dynamic and in vivo mechanical behavior of the lumbar spine during lifting activities. It will pave the way for developing much improved capabilities to assess or predict low back mechanical stress during dynamic manual lifting. It will also afford a unique opportunity to establish new criteria (e.g., in vivo strain-based instead of stress-based) for disc tissue tolerance/failure. This work, along with a series of systematic studies that would naturally follow, will lead to a new body of knowledge and next-generation guidelines, models, and tools for better recognition, evaluation, and control of the risk of occupational low back disorders.